Not your usual cam question (s)
11-29-2009, 03:27 PM
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Staging Lane
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Not your usual cam question (s) Let me start by saying that I do not post a lot, but like to read and follow the forums, but I have never really seen this particular question addressed. I know that this question is in reference to a big block, not LSx but figured it would be best in the FI section.
I feel as though I have a fairly decent understanding of cam specs and basics, but if not, please free to correct me as needed as I will not take offense.
The general trend with aftermarket cams usually (not always, I know, but to some extent) involves increasing overlap along with lift and duration compared to the factory cams, and I understand why that is. It is also not uncommon to either have an advance ground into these cams, or advance them through the use adjustable timing sets and I also understand why this is done. Usually, overlap is decreased in FI setups and this is also varied depending on the type of FI being utilized.
I have seen lobe separations anywhere from 114 to 120+ degrees for turbo grinds, and I also (think) I know the reason(s) for this, but I see very little advance if any ground into these cams. Either this, or when guys are posting the specs on their cams, they are just leaving the advance off. So if a cam has a 116 degree spread, then it should also have a 116 degre ICL as well. However, a 116 degree ICL is also going to reduce low speed torque as well, correct?
It has always been my understanding the the spread was used to help keep the charge from blowing thru the chamber, and that the later closing of the intake valve would also allow to cram more air into the chamber since it is being forced. Which is more important?
Can you have a cam ground with a wide lobe separation, but with advance into it to also help with the bottom end?
The reasons that I am asking these questions are twofold. First off, I am always trying to learn and better my understanding of everything mechanical. Also, I am wanting to turbo my truck (big block, injected) and feel as though I have a pretty good idea of what I will have the cam ground to, but I would also like to have the cam advanced some so that it will a little more low speed torque since it is a heavy truck and I do use it for hauling/towing on occasion.
My truck is a 98 crew cab dually with a 7.4 MPFI. I do not know if I will keep the Vortec intake, but it will not work with the heads I have since the vortec is oval and my heads are rectangle. I am not looking to make this a race truck, but I do feel that mid 14 (or better) second timeslips could be possible with a well thought out combination ... I mean even a brick can fly with enough power, right?
I will be building a 496 stroker and even though I love the sound of a big block with a nice nasty cam, it has to be reasonably smooth since it has to also be the family truckster on occasion. The heads are AFR 305 magnums that I have already have worked and flowed by a friend of mine, and they already have good springs and inconel exhaust valves. I have not made my final decision on the specs, but I am looking something with very fast ramps and good lift. I have looked through the solid roller lobes that Comp Cams has to offer and have found one that is rated at 258 degrees with 230 @ .050 and just under .700 lift with a 1.7 rocker. My springs are good to about 800 lift and have plenty of pressure, especially since I am not wanting to spin the engine too high. I was looking at either a 116 or 118 degree lobe separation, but I have not made any definitive decisions yet.
Should I just have it ground straight up, or can I have it ground with some advance built in? And if I do, will the earlier closing affect the upper power range abnormally? I know that the wider lobe separation generally causes the power curve to continue higher than the same spec cam with a smaller split, but I dont need to really go much over 6K.
Long and short of it, I want the best of both worlds by getting a cam with a wide separation, but advanced to enhance low speed torque as well ... can it work?
Sorry for the short novel, but I was trying to give as much info up front as possible.
I feel as though I have a fairly decent understanding of cam specs and basics, but if not, please free to correct me as needed as I will not take offense.
The general trend with aftermarket cams usually (not always, I know, but to some extent) involves increasing overlap along with lift and duration compared to the factory cams, and I understand why that is. It is also not uncommon to either have an advance ground into these cams, or advance them through the use adjustable timing sets and I also understand why this is done. Usually, overlap is decreased in FI setups and this is also varied depending on the type of FI being utilized.
I have seen lobe separations anywhere from 114 to 120+ degrees for turbo grinds, and I also (think) I know the reason(s) for this, but I see very little advance if any ground into these cams. Either this, or when guys are posting the specs on their cams, they are just leaving the advance off. So if a cam has a 116 degree spread, then it should also have a 116 degre ICL as well. However, a 116 degree ICL is also going to reduce low speed torque as well, correct?
It has always been my understanding the the spread was used to help keep the charge from blowing thru the chamber, and that the later closing of the intake valve would also allow to cram more air into the chamber since it is being forced. Which is more important?
Can you have a cam ground with a wide lobe separation, but with advance into it to also help with the bottom end?
The reasons that I am asking these questions are twofold. First off, I am always trying to learn and better my understanding of everything mechanical. Also, I am wanting to turbo my truck (big block, injected) and feel as though I have a pretty good idea of what I will have the cam ground to, but I would also like to have the cam advanced some so that it will a little more low speed torque since it is a heavy truck and I do use it for hauling/towing on occasion.
My truck is a 98 crew cab dually with a 7.4 MPFI. I do not know if I will keep the Vortec intake, but it will not work with the heads I have since the vortec is oval and my heads are rectangle. I am not looking to make this a race truck, but I do feel that mid 14 (or better) second timeslips could be possible with a well thought out combination ... I mean even a brick can fly with enough power, right?
I will be building a 496 stroker and even though I love the sound of a big block with a nice nasty cam, it has to be reasonably smooth since it has to also be the family truckster on occasion. The heads are AFR 305 magnums that I have already have worked and flowed by a friend of mine, and they already have good springs and inconel exhaust valves. I have not made my final decision on the specs, but I am looking something with very fast ramps and good lift. I have looked through the solid roller lobes that Comp Cams has to offer and have found one that is rated at 258 degrees with 230 @ .050 and just under .700 lift with a 1.7 rocker. My springs are good to about 800 lift and have plenty of pressure, especially since I am not wanting to spin the engine too high. I was looking at either a 116 or 118 degree lobe separation, but I have not made any definitive decisions yet.
Should I just have it ground straight up, or can I have it ground with some advance built in? And if I do, will the earlier closing affect the upper power range abnormally? I know that the wider lobe separation generally causes the power curve to continue higher than the same spec cam with a smaller split, but I dont need to really go much over 6K.
Long and short of it, I want the best of both worlds by getting a cam with a wide separation, but advanced to enhance low speed torque as well ... can it work?
Sorry for the short novel, but I was trying to give as much info up front as possible.
Last edited by hangslo; 11-29-2009 at 03:33 PM.
11-30-2009, 02:53 PM
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wow, your right the ntake wont work. The 496 can probably take a big cam, but I think its more important to match the cam to the weight, convertor and gear. So if its gonna have a 2400 or less stall, (probably will) Id go smaller then your looking. You need to get a lot of weight moving quickly, so you need instant power. I think a 230 at 050 on a 118 will come in late. 112 would be better for that much duration. Im pretty conservative on cams though, and would rather error on the small side. Better drivabilty and less loss in et. Whats the big deal if you shift lower? But if you have to wait to get into power, your sunk on et (ing) well.
11-30-2009, 02:57 PM
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wow, your right the ntake wont work. The 496 can probably take a big cam, but I think its more important to match the cam to the weight, convertor and gear. So if its gonna have a 2400 or less stall, (probably will) Id go smaller then your looking. You need to get a lot of weight moving quickly, so you need instant power. I think a 230 at 050 on a 118 will come in late. 112 would be better for that much duration. Im pretty conservative on cams though, and would rather error on the small side. Better drivabilty and less loss in et. Whats the big deal if you shift lower? But if you have to wait to get into power, your sunk on et (ing) well.
12-05-2009, 07:49 PM
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Staging Lane
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wow, your right the ntake wont work. The 496 can probably take a big cam, but I think its more important to match the cam to the weight, convertor and gear. So if its gonna have a 2400 or less stall, (probably will) Id go smaller then your looking. You need to get a lot of weight moving quickly, so you need instant power. I think a 230 at 050 on a 118 will come in late. 112 would be better for that much duration. Im pretty conservative on cams though, and would rather error on the small side. Better drivabilty and less loss in et. Whats the big deal if you shift lower? But if you have to wait to get into power, your sunk on et (ing) well.
I guess what I am really asking is this: if I have the cam groung with 116 LDA, but with a 2 degree advance, then the intake closing point should be the same as a 112 spread with no advance ground in and this larger spread should act like the narrower grind with respect to low speed torque, correct?
12-07-2009, 05:46 PM
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Staging Lane
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I also need to state that it would be nice to run 14's or better, but not necessary. The situation is this: the truck has low power and gets between 8-10 mpg. I dont mind getting subsatisfactory mileage as long as I have some power to make it fun. I do not drive this truck much ... only about 2K a year. It is my work all, do all, play toy. If I need to pick up two pallets of sod, I can do it ... if I have to go pick up a set of wheels then I can do that and if I finally get my other project done (who am I kidding, started), then I can tow it to car shows and/or the track with the whole family or friends with me.
Anyways, I still havent gotten a definitive answer concerning advancing a wide lobe separation cam for low end torque. I appreciate all the answers so far, but I havent gotten one for this.
Also keep in mind that "coming in late", or having a "weak bottom end" as descriptions for cams also describe the current state of the 7.4 that is in the truck. I believe the specs for the factory cam are around 195/205 on a 116 spread, and it is only rated at 260 hp and 385 ft lbs of torque (peak), so a bigger engine with a softer torque curve on the bottom could in all reality trump what the current engine has to offer.
Anyways, I still havent gotten a definitive answer concerning advancing a wide lobe separation cam for low end torque. I appreciate all the answers so far, but I havent gotten one for this.
Also keep in mind that "coming in late", or having a "weak bottom end" as descriptions for cams also describe the current state of the 7.4 that is in the truck. I believe the specs for the factory cam are around 195/205 on a 116 spread, and it is only rated at 260 hp and 385 ft lbs of torque (peak), so a bigger engine with a softer torque curve on the bottom could in all reality trump what the current engine has to offer.
